The present invention relates to a wireless telecommunication system including at least a base station intended to communicate with terminals over bidirectional communication channels.
Note that wireless telecommunication systems include mobile telecommunication systems in which mobile terminals may move over long distances and sometimes quickly but also telecommunication systems in which the terminals are fixed or may only move over short distances relative to the base station which they are connected to and often very slowly.
FIG. 1 diagrammatically shows a wireless cellular telecommunication system serviced by base stations, here two base stations BTS1 and BTS2, each intended to communicate with at least one terminal, here two terminals TE1 and TE2 communicating with the base station BTS1 over wireless communication channels CH1 and CH2, respectively. At the moment of the FIG. 1, the terminal TE1 is at a distance d1 from the base station BTS1 whereas the terminal TE2 is at a distance d2 from the base station BTS1. The area covered by a base station BTS is generally called a cell, the border of said cell being at a distance of the base station considered as maximal.
Each channel CHi (i=1, 2) is intended to support an uplink ULi for carrying information from the terminal TEi to the base station BTS and a downlink DLi for information from the base station BTS to the terminal TEi. Said information is enclosed within frames split into time slots allocated either to the uplink ULi or to the downlink DLi.
The frame is for example of the type depicted in FIG. 2, i.e. of the HD/OFDM type (standing for Half Duplex/Orthogonal Frequency Division Multiplex/OFDM) either TDD/OFDM (Time Division Duplex/OFDM) or FDD/OFDM (Frequency Division Duplex). As it can be seen at FIG. 2, this frame is subdivided into an integer number L of time slots TS1 to TSL that can be allocated either to the downlink DL or to the uplink UL. Furthermore, each time slot TSj (j=1 to L) supports qj symbols s1 to sqj (here, for the time slot TSj, qj=8), called OFDM symbols, respectively carried by k orthogonal modulation frequencies f1 to fk. Note that each of the OFDM symbols s1 to sqj in a time slot TSj generally includes a cyclic prefix that is used to combat inter-symbol interference.
It must be understood that in a general case the number of symbols per time slot can vary from a time slot to another.
In relation with FIG. 3, let's consider the transmission at time te of qj=eight symbols s1 to s8 over the downlink DL by a base station BTS. These symbols s1 to s8 are received by a terminal TEref at the border of the considered cell (at a distance dref from the base station BTS) at a time equal to te+RTD(dref)/2 (RTD(dref) being the Round Trip Delay for that terminal TEref at said distance dref from the base station BTS). These symbols are processed by the terminal which then transmits also symbols over the Uplink UL. Before transmitting over the uplink, a terminal has to wait for a period of time, said Receive Transmit Switch time or simply switching time and referred to as RTS, in order to take into account the duration of hardware and software operations. For instance, this delay RTS is the maximum of the time needed by hardware equipments of the terminals to switch between reception and transmission and the time needed by hardware equipments of the base station to switch between transmission and reception. The symbols transmitted over the uplink UL are received at the base station BTS at a time tr equal to te+RTD(dref)+RTS+DDL, DDL being the total duration of the qj symbols. It can thus be seen that the base station BTS has to wait for the reception of the symbols transmitted by a terminal located at the border of the cell in order to perform the processing thereof. The waiting time is called the Guard Period GP and must be equal at least to the round trip delay RTD(dref) plus the Receive Transmit Switch time RTS.
Guard periods GP between downlinks DL and uplinks UL can be seen on FIG. 2.
For those terminals that are closer to the base station BTS than the one TEref above considered, a timing delay TD(d) is generally determined and applied for the transmission of symbols over the uplink UL in such a manner that the transmitted symbols are received at the base station BTS from all the terminals TE connected thereto at the same time tr. In other words, the guard period GP is constant whatever the distance separating the considered terminal from the base station BTS.
Note that in the litterature, the notion of timing advance TA is well-known. The timing delay TD as aforedefined can be linked to the timing advance TA by considering the guard period GP as follows:TA=GP−TD 